The fields of medical and industrial radiography use the technique of directing beams of electromagnetic radiation toward an object (or part of the human body), so that the radiation passes through the object, to obtain an image of the interior of the object, that is otherwise difficult to access or view directly without cutting through the body or other object. Usually, the electromagnetic radiations used for imaging purposes are x-rays, which tend to scatter as they travel through the object to be imaged.
The scattered x-rays contribute to the degradation of the image of the object and more particularly to the degradation of the image contrast. The x-rays that travel through the object that are not scattered are referred to as primary transmissions and it is those transmissions that contribute the most useful information to the image. The various unscattered x-rays passing though the object are attenuated at differing levels by differing amounts and compositions of material within the object. The differences in x-ray attenuation along linear paths through the object produce an intensity pattern that comprises image information which is recorded by an image receptor.
The image receptor may be a screen having a layer of x-ray sensitive material or x-ray sensitive electronic medium. The resulting image produced by the image receptor is based on the differences in the intensity of primary x-ray transmissions detected by the receptor. To improve the image quality, the primary x-ray transmissions and any scattered x-rays that would reach the image receptor after having traveled though the body, are first passed through a grid before they are allowed to impinge onto the image receptor.
It is understood that, quantitatively speaking, the scattered x-rays degrade the image contrast by a factor approximately equal to (1-SF) where SF is the scatter fraction of the total radiation transmitted through the body. The scatter fraction SF is defined as:
  SF  =      S          S      +      P      where S and P are the intensities of the scattered and primary radiations incident on the image receptor, respectively.